In recent years, a projection type video display apparatus which can project a favorable large projection image even at a short distance has been widely used. In order to achieve the projection of the favorable large projection image, application of so-called oblique projection optical system is already known in a projection optical system of such a projection type video display apparatus, the oblique projection optical system using a free-form surface lens in combination with a concave mirror having an eccentric aspherical shape.
For example, Patent Document 1 discloses a projection optical apparatus which includes a projection lens constituted by a first optical system including a transmission type refractive element and a second optical system including a reflection type refractive element, and in which some lenses of the first optical system is housed in a lower space whose lower limit is at a lower end of the second optical system.
In addition, Patent Document 2 discloses a projection type video display apparatus which can project a favorable large projection image even at a short distance by using a combination of a lens system and a concave mirror.